The purpose of these studies is to develop methods for non-invasive evaluation of perfusion and blood oxygenation, and to perfect real- time imaging of rapidly moving organs such as the heart. The basic experimental tool is Echo-Planar Imaging (EPI), a type of magnetic resonance imaging which acquires images in 0.1 s or less, much faster than conventional MRI. The images reflect changes in blood oxygenation, since deoxyhemoglobin is highly paramagnetic. The images can be made sensitive, through the use of injected intravascular contrast agents, to spatial variations in blood flow. In the past year we have mainly been engaged in studies of blood oxygenation in cat brain and functional activity in human brain. At a magnetic field of 2 T, we have used a cat respiratory challenge model to observe the time course of imbalances of oxygen supply and utili-zation in brain tissue, and using the whole-body 4 T magnet recently acquired by the Laboratory we have performed experiments studying the localization of visual and motor function in the brains of human vol- unteers. The following findings have been made. 1) For constant blood volume, there is a linear relationship between the change in relaxa-tion rate observed in gradient echo MR images of cat brain and the change in blood hemoglobin saturation caused by respiratory chal-lenges. 2) Active regions in primary visual and motor cortex in human volunteers can be identified using gradient-echo echo-planar images. Photic stimulation results in changes of image intensity in highly focal regions in the primary visual cortex of up to 25 percent.